In optical information recording media including a CD and a DVD, an organic dye material has mainly been used as a write-once optical information recording medium material so far. The reason is that when the organic dye material is used, it is relatively easy to sustain interchangeability with a read-only memory (ROM), for which a relatively high reflectance is requested under the standards, over a wavelength band of laser light that is used for recording or reading. Moreover, the recording medium can be produced through a simple process of forming a reflective layer according to a sputtering method after spin-coating the organic dye. The use of the organic dye material is advantageous in terms of a cost of manufacture such as plant investment.
However, since the wavelength of laser light to be employed in recording or reading has gotten shorter and laser light in a wavelength band of blue violet (wavelength of about 400 nm) has come to be used, circumstances become different. Specifically, it is not easy to prepare an organic dye, which can cope with the laser light of the wavelength, in terms of recording sensitivity and a signal property. The layer construction of a recording medium cannot be readily accomplished through the conventional simple process. Further, the fact that when the recording medium is manufactured according to the spin coat method, the homogeneity of a land part with a groove part cannot be retained has been found apparently disadvantageous in terms of a push-pull signal or crosstalk in high-density recording.
In order to meet the requirements, the employment of an inorganic recording material instead of the organic dye material is currently encouraged in earnest. The practical use of a recording medium employing the inorganic recording material has been shelved because the recording medium is hardly interchangeable with the ROM of a high reflectance and an expensive sputtering apparatus is needed to form many layers of the material, though the recording medium has been studied in the past. However, the dependency of the inorganic recording material on the wavelength of employed laser light is generally not as high as that of the organic material is. Moreover, formation of many recording layers that has often been performed in recent years (inclusion of many recording layers makes the recording capacity of an optical disk of the same size two or more times larger) can be more readily achieved than employment of the organic dye material. Therefore, the use of the inorganic recording material as a new-generation optical recording material in place of the organic dye material has become a mainstream, and the inorganic recording material has been put to practical use.
As the inorganic recording material, various types of materials have been proposed in the past. For example, an optical recording medium employing a recording layer that has two or more layers of thin films made of different metallic materials joined has been proposed (refer to a patent document 1). In the optical recording medium, a multilayer film is partly recomposed into an alloy to form a single film using heat dissipated with irradiation of laser light in order to form record marks. As applied examples of this method, various forms have been proposed with the material varied differently (refer to, for example, a patent document 2). Moreover, a write-once optical recording medium using an oxide compound for the recording layer has been proposed (refer to, for example, patent documents 3 and 4).
However, the foregoing recording media cannot be said to fully satisfy all conditions required for the write-once optical information recording medium. Specifically, required are that information recorded in the write-once optical information recording medium is preserved stably over a long period as it initially is (archival property), that a signal is not impaired by reading laser light during signal reading (reading stability), and that the writing property is sustained without degeneration caused by normal long-term preservation (shelf-life property). The aforesaid conventional recording media cannot be said to satisfactorily have these properties. Moreover, from the viewpoints of a cost of manufacture of a recording medium or reserve of a margin in a manufacturing process, the number of layers constituting a recording medium is requested to be as small as possible, and the manufacturing process is requested to be simple. Moreover, sufficient sensitivity and a sufficient response speed are desired in terms of recording and reading properties. Thus, an excellent recording/reading signal is guaranteed for a wide range of linear velocities.
Proposed as an optical recording medium employing an inorganic recording material are: a medium having a recording layer made of an alloy containing Zn, Sn, Sb, and Te (refer to a patent document 5); a medium having a recording layer made of a recording material that contains as a principal component a compound expressed as AxB1-x (where A denotes at least one element selected from a group of Zn, Ga, In, Si, Ge, Sn, Bi, and Sb, B denotes at least one element selected from a group of Se, Te, S, and O, and x denotes a compositional ratio determined with a range (2.0 to 3.0) of values of a mean coordination number) (refer to a patent document 6); a medium having a recording layer formed with an alloy thin film expressed as Mw((SbzTe1-z)1-w(0≦w<0.3, 0.5≦z<0.9, and M denotes at least one element selected from a group of In, Ga, Zn, Ge, Sn, Si, Cu, Au, Ag, Pd, Pt, Pb, Cr, Co, O, S, and Se) (refer to patent documents 7 and 8); a medium having a recording layer made of one material that contains Te as a principal component, contains as a secondary component an alloy that contains at least one element, which is selected from a group of Ge, Sb, Bi, Se, S, As, Tl, In, Ga, Au, Ag, Cu, Pd, Pt, Ni, and Co, by an atomic percent equal to or larger than 30 and smaller than 60 in total, and contains as an additive at least one material selected from a group of ZnS, ZnTe, ZnSe, SiO2, TiO2, Al2O3, TiC, ZrC, and HfC (refer to a patent document 9); a medium including a recording layer that has a composition expressed as SbaInbSncZndSieOfSh where a>0, b>0, c>0, d>0, e>0, f>0, h>0, and a+b+c+d+e+f+h=100 are met (refer to a patent document 10); a medium including a recording layer that has a composition expressed as SbaXbSncZndSieOfSh where X denotes an element selected from among In, Ge, Al, Zn, Mn, Cd, Ga, Ti, Si, Te, Nb, Fe, Co, W, Mo, S, Ni, O, Se, Tl, As, P, Au, Pd, Pt, Hf, and V, and a>0, b>0, c>0, d>0, e>0, f>0, h>0, and a+b+c+d+e+f+h=100 are met (refer to a patent document 11); and a medium including a recording layer that contains at least one metal M selected from a group of Ni, Cu, Si, Ti, Ge, Zr, Nb, Mo, In, Sn, W, Pb, Bi, Zn, and La, and an element X which binds with the metal M when being irradiated a recording laser beam so as to produce a crystal of a compound with the metal M (refer to patent documents 12 and 13).
Patent document 1: JP-A-62-204442
Patent document 2: Japanese Patent No. 3066088
Patent document 3: JP-B-54-7458
Patent document 4: JP-A-2006-281751
Patent document 5: JP-A-11-235873
Patent document 6: JP-A-8-104060
Patent document 7: JP-A-10-172179
Patent document 8: JP-A-2001-236690
Patent document 9: JP-A-5-124353
Patent document 10: JP-A-2003-72244
Patent document 11: JP-A-2003-182237
Patent document 12: JP-A-2005-125726
Patent document 13: JP-A-2005-129192